Kinetic Simulation of the Electron-Cyclotron Maser Instability: Relaxation of Electron Horseshoe Distributions

摘要

The electron-cyclotron maser instability (ECMI) is responsible for generationof the planetary auroral radio emissions. Most likely, the same mechanismproduces radio bursts from ultracool dwarfs. We investigate amplification ofplasma waves by the horseshoe-like electron distribution (similar to thoseobserved in the terrestrial magnetosphere) as well as relaxation of thisdistribution due to the ECMI. We aim to determine parameters of the generatedplasma waves, timescales of the relaxation process, and the conversionefficiency of the particle energy into waves. We have developed a kineticrelativistic quasi-linear 2D code for simulating the coevolution of an electrondistribution and the high-frequency plasma waves. The code includes theprocesses of wave growth and particle diffusion which are assumed to be muchfaster than other processes (particle injection, etc.). A number of simulationshave been performed for different parameter sets which seem to be typical forthe magnetospheres of ultracool dwarfs (in particular, the plasma frequency ismuch less than the cyclotron one). The calculations have shown that thefundamental extraordinary mode dominates strongly. The generated waves have thefrequency slightly below the electron cyclotron frequency and propagate acrossthe magnetic field. The final intensities of other modes are negligible. Theconversion efficiency of the electron energy into the extraordinary waves istypically around 10%. Complete relaxation of the unstable electron distributiontakes much less than a second. Energy efficiency of the ECMI is more thansufficient to provide the observed intensity of radio emission from ultracooldwarfs. On the other hand, the observed light curves of the emission are notrelated to the properties of this instability and reflect, most likely,dynamics of the electron acceleration process and/or geometry of the radiationsource.